U.S. Pat. No. 5,315,139 corresponding to JP 3207615 discloses a scroll-shaped field plate (SRFP) for reducing an electric field in a lateral high-voltage lateral power element. The SRFP is used for a high-voltage lateral diode, a high-voltage lateral IGBT, and a high-voltage lateral MOSFET. The SRFP allows such an element to have a desired breakdown voltage for direct current and low speed switching application.
U.S. Pat. No. 5,315,139 is silent on a resistance of the SRFP. The present inventors deeply studied a relationship between the resistance of the SRFP and the breakdown voltage and found that the breakdown voltage can vary depending on the concentration of an impurity in the SRFP and the resistance of the SRFP.